Cells may include one or more sectors. A cell without multiple sectors is a single sector cell, i.e., it includes a single sector. Signals are normally transmitted by a sector transmitter using a carrier frequency and the corresponding bandwidth, e.g., one or more tones surrounding the carrier frequency. Different cells and/or sectors of a cell often use different frequency bands centered around the carrier frequencies used by the sectors or cells. In a frequency reuse system, the carrier frequencies of adjacent cells and/or sectors are often different. To receive signals corresponding to a carrier frequency, a wireless terminal normally has to adjust its receiver, e.g., receiver filters, to correspond to the frequency band associated with the carrier frequency to be used. Switching a receiver between carrier frequencies may take time. Thus, in receivers with a single filter chain, transitioning between different carriers may cause the receiver to encounter intervals during which information can not be received due to the switching process.
Wireless terminals, e.g., mobile nodes, communicating with a base station on a given carrier frequency and moving through a multi-carrier system need to decide when to make a handoff and transition to a new carrier frequency, e.g., corresponding to a new cell and/or sector. As discussed above, an adjacent sector and/or cell may use a different carrier frequency, and as a sector or cell boundary is crossed, a wireless terminal will normally have to identify and switch to the new carrier frequency.
Typically a mobile node, listens to one carrier frequency band at a given time due to constraints in the hardware and cost associated with the receiver. This is because, for cost reasons, multiple parallel receiver filter chains are often too expensive to be practical. In some known systems a mobile node waits until communications are lost or significantly degraded on the operating carrier band being used before switching to another carrier. In some systems, a wireless terminal periodically switches its receiver to a different carrier band to check for signal presence and/or strength. Unfortunately, while switched to search for another carrier, the receiver can not receive signals from the carrier that is currently in use. The known methods of determining what carriers are available to switch to and when to switch to a new carrier may result in interrupted communications, gaps during the hand-off process, and/or wasted resources in monitoring and determining the appropriate carrier frequency band.
In view of the above discussion, it should be appreciated that there is a need for improved methods for determining when a wireless terminal should initiate a handoff. Preferably, any new or improved methods should not require a mobile node to switch its receiver to another frequency band to search for the carrier frequency of an adjacent cell or sector.